Errr... He is fighting someone that can turn part of his body into a bear, thats not very natural physics either
Well...
John's hammers are rather long (longer than himself), so he may not be able to gain enough leverage for a solid strike.
It really depends if Senri keeps him at arm's length, where John has the advantage, or pulls him in for a good bear mauling.
John could use the Wrinklefucker's heat to discourage Senri from staying close, but that alone would not do enough damage to really inflict serious wounds.
The wrinklefucker MAY do heat damage, but its unknown, and getting hit by a large chunk of metal is probably much mroe damaging then some steam.
When using the wrinklefucker he would have no problem getting leverage, as it appears to get something like 100% bounce, so in a few seconds he could speed it up fast enough that any hit would be very hard. If senrei rushed him he might not be able to get leverage on the fear no anvil, but john could just switch to the wrinklefucker.
And senrei couldn't maul john, he only has 1 bear arm.
Case for Senri:
Yes, Senri is strong, about as strong as a bear. But he has other things going for him too. For one thing, he is great at surviving in wilderness areas. He is also taller than john. Even though he doesn't talk much, he is also quite smart on some subjects. Senri will be able to make traps, and use them to attempt to catch john or an animal to eat. If he does have to fight john he will probably not be able to dodge all of john's attacks, but he also carries around a falchion which he should be able to use with his other arm to try to block john's attacks. Also, since climbing the Echeladder seems to give john more health, which level of the Echeladder will he be on?
He will be wherever on the echeladder he was at the time of this discussion, however no matter where he is because its kinda arbitrary to us since we don't know how big the effects are. As for food, john could carry 20 steak meals with him without any trouble (in his inventory system), or probably like 100 cans of beans and a food heater.
Don't think Senrei would be able to do any traps that would have a real effect on john, since he has a jetpack (to get out of holes or a noose trap on his foot), and super health (for any trap that would hurt him).